Nuclear receptors program many endocrine processes essential for health and life. Among the most enigmatic are those receptors, which are critically important but have unknown regulating hormones, such as DAX-1, LRH-1 and SF-1. These are the targets of this proposal. Medical disorders teach us about the interplay of these receptors in the endocrine system without telling us their mechanisms. The DAX-1 gene codes an unusual nuclear receptor lacking a DNA interaction domain. Yet it controls the activity of many target genes in many tissues. Mutations in DAX-1 affect embryonic stem cell progression, development of the adrenal and pituitary glands, the hypothalamus and the ovaries or testes leading to severe endocrine disorders. The receptors LRH-1 and SF-1 are critical in developmental processes, and their constitutive activation is muted by DAX-1. These three nuclear receptors and transcription factors are called orphans because their controlling hormones, if they exist, have not been identified. The most basic details of the mechanisms of regulation of these related nuclear receptors are not known. The goal of this proposal is to understand the principles and atomic level details of the mechanisms of regulation of LRH-1 and SF-1 by DAX-1. Since it halts transcriptional activation, DAX-1 may be classed as a co-repressor with unknown hormone. Because regulation by DAX-1 depends on the associations of these proteins we will analyze, at atomic resolution, the binding interactions of DAX-1 with LRH-1 and SF-1. DAX-1 protein, which has never been prepared in a functionally active state for either biochemical or structural studies, will be isolated and crystallized with LRH-1 and SF-1 so that three-dimensional structures of the assemblies may be analyzed. Functional studies guided by the structural models will carried out in cells to reveal the mechanisms of action of these receptors. This information will be the foundation for understanding the molecular pathology of numerous human endocrine syndromes and suggest pathways for therapeutic intervention. The nuclear receptors we are studying are associated with many disease processes, including cancer, infertility, genetic metabolic diseases, and mental health defects. Learning the mechanisms of action of these receptors, and what goes wrong in the diseases, will help suggest pathways for design of new drug treatments for these conditions. Project Narrative The nuclear receptors we are studying are associated with many disease processes, including cancer, infertility, genetic metabolic diseases, and mental health defects. Learning the mechanisms of action of these receptors, and what goes wrong in the diseases, will help suggest pathways for design of new drug treatments for these conditions.